1. Field of the Invention
The present invention relates to a VCO nonlinear compensation system for compensating a nonlinearity of a voltage-controlled oscillator (VCO).
2. Description of the Background Art
Conventionally, various methods of causing an oscillatory frequency response of a VCO to be changed linearly have been used. FIG. 8 is a diagram illustrating a configuration of a conventional direct synthesis modulator 600, for generating a communication carrier frequency, which is disclosed in U.S. Pat. No. 7,061,341.
The conventional direct synthesis modulator 600 has a PLL circuit including a voltage-controlled oscillator 604, an N-divider (N counter) 606, a phase comparator 608, a charge pump (CP) 616 and a RC coupling filter.
A phase signal 610 corresponding to a desired channel is converted by a delta-sigma modulator of a digital differential circuit 612 into a digital signal, and then further converted by a D/A converter into an analog signal. Thereafter, the analog signal is inputted to an auxiliary terminal 602 of the voltage-controlled oscillator 604 via an LPF 614. On the other hand, a modulated signal outputted from the digital differential circuit 612 is supplied to the N-divider 606 after being matched with a data rate of a control signal.
With such a configuration, a signal obtained by dividing an output of the voltage-controlled oscillator 604 by N is compared with the modulated signal in the phase comparator 608 and a feedback on a difference between the two signals is provided, thereby compensating for a nonlinearity of the voltage-controlled oscillator 604.
However, the auxiliary terminal 602 of the voltage-controlled oscillator 604 has a function of controlling not a phase but an oscillatory frequency. Therefore, a phase modulator is required for shifting a phase of the voltage-controlled oscillator 604.
Furthermore, a D/A converter or a down-converter, for example, is required for providing the feedback. As a result, a circuit becomes complicated and the number of components included in the circuit is increased, whereby it is difficult to adjust a timing of the feedback.